Thread: Quantum Computers

Heh, I was way off.

But give me some time. I'm only in first year engineering and I plan on getting into Multidisciplinary Engineering Physics. I'll be covering nanotechnology in that.

Computing with Molecules. In addition to nanotubes, major progress has been made in recent years in computing
with just one or a few molecules. The idea of computing with molecules was first suggested in the early 1970s by
IBM's Avi Aviram and Northwestern University's Mark A. Ratner.14 At that time, we did not have the enabling
technologies, which required concurrent advances in electronics, physics, chemistry, and even the reverse engineering
of biological processes for the idea to gain traction.
In 2002 scientists at the University of Wisconsin and University of Basel created an "atomic memory drive" that
uses atoms to emulate a hard drive. A single silicon atom could be added or removed from a block of twenty others
using a scanning tunneling microscope. Using this process, researchers believe, the system could be used to store
millions of times more data on a disk of comparable size—a density of about 250 terabits of data per square inch—
although the demonstration involved only a small number of bits.15
The one-terahertz speed predicted by Peter Burke for molecular circuits looks increasingly accurate, given the
nanoscale transistor created by scientists at the University of Illinois at Urbana-Champaign. It runs at a frequency of
604 gigahertz (more than half a terahertz).16
One type of molecule that researchers have found to have desirable properties for computing is called a
"rotaxane," which can switch states by changing the energy level of a ringlike structure contained within the molecule.
Rotaxane memory and electronic switching devices have been demonstrated, and they show the potential of storing
one hundred gigabits (1011 bits) per square inch. The potential would be even greater if organized in three dimensions.

From Ray Kurzweils book "The Singularity is Near" (2005).

If Moore's law continues, computers'll have the power of all of the brains of the world population by 2050, if I remember rightly.

I don't know anything about quantum computing unfortunately... wish I did, it sounds pretty interesting.

Originally Posted by AirRick101

how big is your brain, switch?

Thanks for the compliment AirRick and that’s a really good question. My colleagues tell me I think way too much. I’ve been that way since I was a little girl. I guess that’s the reason why I selected the filed that I’m in now.

Originally Posted by Sound

From Ray Kurzweils book "The Singularity is Near" (2005).

I’m glad you’ve brought this up because I’ve actually read this book, it took me over a year to complete it but I’ll say he is extremely optimistic and it’s very enlightening. There were some boring bits however it is definitely worth the time invested that’s for sure. I will say this regarding population growth. When millions of humans can exist as uploaded brains in a computer the size of a rock, population growth is no longer a problem. Besides we’ll just colonize space when we have AI smart enough to figure out how it can be done, unless we figure out immortality before anything else.

If you get a chance to read The Age of Spiritual Machines, you will not be disappointed. Pay extremely close attention to the second edition, which looks at where we are now, the choices we, face, and the forces that will continue to drive technology's advance. I was most impressed with his argument that we will soon implement various advances, simply because they make life better. For example: If you could dictate (with 100% reliability) into your computer rather than typing much more slowly wouldn't you? Of course you would. How about a more direct brainwave link, so you could even faster simply think your words onto the screen. Would you want that? Absolutely! And so it goes, with the man-machine link becoming ever more intimate. And that trend will go on and on simply because people will “WANT” it to. He makes a very arresting point, along the way (Moore's Law): Computers are increasing in power, doubling their "intelligence" approximately every two years. In short his arguments are very compelling regarding why the trends will be durable rather than dying away. Bottom line: I was convinced.

Originally Posted by Xei

If Moore's law continues, computers'll have the power of all of the brains of the world population by 2050, if I remember rightly.

I don't know anything about quantum computing unfortunately... wish I did, it sounds pretty interesting.

I agree. It makes perfect sense.

I was also really looking forward to seeing some feedback from one of these guys.

Ynot
Ninja9578
Marvo
Elis D.
DSR

Quantum computers are 4 states rather than two, this is what makes them so powerful. A q-bit can be on, off, neither, or both.

IBM has a 28 q-bit machine working right now, but it's very slow going from 28 to 29 because of something called the Heisenburg Uncertainty Principal. The problem is that states aren't constant. Once you read the state of a q-bit, it changes, so you have to get around that, which requires a lot of really weird physics. It'll be decades before we see a working quantum machine.

If I remember right this was based around the fact that a quantum system can hold more than one state at the same time: superpositioning.


Quantum physics really goes over my head, but in terms of feasibility, it's been demonstrated that quantum computers work in principal already. Whether it could ever be sped up to a level where it dwarfs all computing power we have today, who knows? But with our ever-growing and increasingly rapid gaining of new technologies and knowledge, who knows what might be possible?

My colleagues tell me I think way too much

Nothing wrong with that. We need more people thinking more

Originally Posted by Sound

From Ray Kurzweils book "The Singularity is Near" (2005).

Note that this excerpt is talking not about quantum computing, but ordinary, classical computing on a really small scale.

Quantum computers are 4 states rather than two, this is what makes them so powerful. A q-bit can be on, off, neither, or both.

The following assumes that the modern standard model holds. If it doesn't, all bets are off.

IIRC, this is not exactly true. A qubit (quantum bit) doesn't simply have four distinct states of off/on/both/neither. For starters, it can't really be neither. It can be in states representing 0 or 1, or, crucially, in a quantum superposition of both states. Wikipedia probably explains this better than I can, but it means that the state of the qubit in a superposition of states 0 and 1 can be thought of as a a combination of each state multiplied by its respective probability amplitude. The value of these probability amplitudes are complex numbers with the sum of the squares of the moduli being equal to 1.

However, this doesn't mean that a qubit is like a regular bit, but with more states. Qubits are a lot more complicated to manipulate and read.

For example: Measuring a qubit will not simply reveal the probability amplitudes. You'll actually measure the state of it as either 0 or 1, with the probability of each being equal to the square of the modulus of its probability amplitude (this is why these values need to sum to 1).

Quantum computation is really interesting.

Originally Posted by archdreamer

The value of these probability amplitudes are complex numbers with the sum of the squares of the moduli being equal to 1.

Actually the probability amplitudes are real numbers and are calculated as the mod-squared of the wavefunction, which is complex. /end nitpick

But yeah, I don't think quantum computers will ever be a replacement for classical computers because of the way they work. The programming is essentially the hardware itself, and it finds the answer by settling into the most probable set of positions. The areas will quantum computers will be useful are when a MASSIVE NP calculation has to be done that would take a classical computer weeks or more to do. Then it's worth building a quantum computer to do that calculation.

The biggest problem is that even if quantum chips are faster than regular ones, it doesn't really matter. Chips have reached the limit of how fast they can go because they are dependent on RAM. CPUs have gotten so fast that they now have to slow down and wait for the RAM.

Wanna make a billion dollars? Invent a superfast RAM chip.

I just got back from visiting my friend at Cornell, she and her PhD adviser are currently doing research on M-RAM. It's fast but they can't keep it stable outside of super cold temperatures.

Originally Posted by drewmandan

Actually the probability amplitudes are real numbers and are calculated as the mod-squared of the wavefunction, which is complex. /end nitpick

Are you sure? From my understanding, the value of the wavefunction (function that maps from a space representing the possible states of the system, to the complex numbers) at any particular point is the probability amplitude (complex), and squaring the modulus of that will give you the probability of the system being measured as being in that state (real). What you describe sounds like this value of probability. Call me on this if I am wrong, though, I'm no quantum physicist, just an interested amateur.

The biggest problem is that even if quantum chips are faster than regular ones, it doesn't really matter [because of current RAM speeds]

This is way off, as far as I know. Quantum computing doesn't rely on traditional RAM. You initialise the system, allow its components to interact, then measure some of them (like a regular computer, dohohoho). You might use traditional RAM to store input and output data, but it isn't going to significantly impact the speed of your quantum computation. You may need, however, some sort of quantum analogue to ordinary RAM, but the design of something like that would be very different to the design of RAM today.

Originally Posted by archdreamer

Are you sure? From my understanding, the value of the wavefunction (function that maps from a space representing the possible states of the system, to the complex numbers) at any particular point is the probability amplitude (complex),

It's not probability, it's the wavefunction amplitude. Probability is a real number between 0 and 1.

Originally Posted by archdreamer

and squaring the modulus of that will give you the probability of the system being measured as being in that state (real). What you describe sounds like this value of probability. Call me on this if I am wrong, though, I'm no quantum physicist, just an interested amateur.

Probability amplitude is a different term for probability. Same thing.

Originally Posted by drewmandan

Probability amplitude is a different term for probability. Same thing.

I hate to be that guy, but, I don't think this is the case. Sources follow.

Probability amplitude: A complex number, the squared norm of which gives a probability.

this URL is long, it's a physics dictionary on google books

The probability distribution itself is obtained by squaring the probability amplitude.

http://www.chemistry.mcmaster.ca/esa...section_2.html

Note that this probability amplitude is quite distinct from the probability itself; to repeat, the probability is proportional to the modulus squared of the probability amplitude.

QM textbook on google books, awful scan

Originally Posted by archdreamer

Are you sure? From my understanding, the value of the wavefunction (function that maps from a space representing the possible states of the system, to the complex numbers) at any particular point is the probability amplitude (complex), and squaring the modulus of that will give you the probability of the system being measured as being in that state (real). What you describe sounds like this value of probability. Call me on this if I am wrong, though, I'm no quantum physicist, just an interested amateur.



This is way off, as far as I know. Quantum computing doesn't rely on traditional RAM. You initialise the system, allow its components to interact, then measure some of them (like a regular computer, dohohoho). You might use traditional RAM to store input and output data, but it isn't going to significantly impact the speed of your quantum computation. You may need, however, some sort of quantum analogue to ordinary RAM, but the design of something like that would be very different to the design of RAM today.

They would have the same implementation, but they would be qubits instead of bits. They still need a way to increase the bus speed though, that's what is restraining computer speed right now. I don't think the laws of quantum physics allow for quantum RAM anyway, it's too volatile and the information will degrade too quickly.

Quantum CPU speeds will not matter, even for huge calculations if the RAM bus speed is still slow. Most CPUs have 32 registries and about 32MB of cache, huge calculations require gigabytes to do, so it'll have to swap to quantum RAM a lot, which restricts the speed of the calculations.